Inoculation (also known as variolation) was a historical method for the prevention of smallpox by deliberate introduction into the skin of material from smallpox pustules. This generally produced a less severe infection than naturally-acquired smallpox, but still induced immunity to it. The term entered medical English through horticultural usage meaning to graft a bud (or eye) from one plant into another. It is derived from the Latin in + oculus (eye).[1] Though innoculation/innoculate is sometimes seen, this is incorrect, possibly erroneously thought to be related to innocuous, which is derived from the Latin in + nocuus (not harmful).

The terms inoculation, vaccination, immunization and injection are often used synonymously to refer to artificial induction of immunity against various infectious diseases. This is supported by some dictionaries.[1][2][3] However, there are some important historical and current differences. In English medicine inoculation referred only to the prevention of smallpox until the very early 1800s. When Edward Jenner introduced smallpox vaccine in 1798 this was initially called cowpox inoculation or vaccine inoculation. Soon, to avoid confusion, smallpox inoculation was referred to as variolation (from variola = smallpox) and cowpox inoculation was referred to as vaccination (from Jenner's use of Variolae vaccinae = smallpox of the cow). Then, in 1891 Louis Pasteur proposed that the terms vaccine/vaccination should be extended to include the new protective procedures being developed. Immunization refers to the use of all vaccines but also extends to the use of antitoxin, which contains preformed antibody to e.g. diphtheria or tetanusexotoxins. Inoculation is now more or less synonymous in nontechnical usage with injection etc., and the question e.g. 'Have you had your flu injection/vaccination/inoculation/immunization?' should not cause confusion. The focus is on what is being given and why, not the literal meaning of the technique used.

Inoculation also has a specific meaning for procedures done in vitro. These include the transfer of microorganisms into and from laboratory apparatus such as test tubes and petri dishes in research and diagnostic laboratories, and also in commercial applications such as brewing, baking and the production of antibiotics.

In almost all cases the material inoculated is called the inoculum, or less commonly the inoculant, although the term culture is also used for work done in vitro.

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The earliest hints of the practice of inoculation for smallpox in China come during the 10th century.[4] A Song Dynasty (960–1279) chancellor of China, Wang Dan (957–1017), lost his eldest son to smallpox and sought a means to spare the rest of his family from the disease, so he summoned physicians, wise men, and magicians from all across the empire to convene at the capital in Kaifeng and share ideas on how to cure patients of it until the gods sent a divine man from Mt. O-mei who carried out inoculation. However, the sinologist Joseph Needham states that this information comes from the Zhongdou xinfa (種痘心法) written in 1808 by Zhu Yiliang, centuries after the alleged events.[4]

The first clear and credible reference to smallpox inoculation in China comes from Wan Quan's (1499–1582) Douzhen xinfa (痘疹心法) of 1549, which states that some women unexpectedly menstruate during the procedure, yet his text did not give details on techniques of inoculation.[5] Inoculation was first vividly described by Yu Chang in his book Yuyi cao (寓意草), or Notes on My Judgment, published in 1643. Inoculation was reportedly not widely practised in China until the reign of the Longqing Emperor (r. 1567–1572) during the Ming Dynasty (1368–1644), as written by Yu Tianchi in his Shadou jijie (痧痘集解) of 1727, which he alleges was based on Wang Zhangren's Douzhen jinjing lu (痘疹金鏡錄) of 1579.[5] From these accounts, it is known that the Chinese banned the practice of using smallpox material from patients who actually had the full-blown disease of Variola major (considered too dangerous); instead they used proxy material of a cotton plug inserted into the nose of a person who had already been inoculated and had only a few scabs, i.e. Variola minor. This was called "to implant the sprouts", an idea of transplanting the disease which fit their conception of beansprouts in germination. Needham quotes an account from Zhang Yan's Zhongdou xinshu (種痘新書), or New book on smallpox inoculation, written in 1741 during the Qing Dynasty (1644–1912), which shows how the Chinese process had become refined up until that point:

Method of storing the material. Wrap the scabs carefully in paper and put them into a small container bottle. Cork it tightly so that the activity is not dissipated. The container must not be exposed to sunlight or warmed beside a fire. It is best to carry it for some time on the person so that the scabs dry naturally and slowly. The container should be marked clearly with the date on which the contents were taken from the patient.

In winter, the material has yang potency within it, so it remains active even after being kept from thirty to forty days. But in summer the yang potency will be lost in approximately twenty days. The best material is that which had not been left too long, for when the yang potency is abundant it will give a 'take' with nine persons out of ten people—and finally it becomes completely inactive, and will not work at all. In situations where new scabs are rare and the requirement great, it is possible to mix new scabs with the more aged ones, but in this case more of the powder should be blown into the nostril when the inoculation is done.[5]

According to Voltaire (1742), the Turks derived their use of inoculation to neighbouring Circassia.

The Circassian women have, from time immemorial, communicated the small-pox to their children when not above six months old by making an incision in the arm, and by putting into this incision a pustule, taken carefully from the body of another child. This pustule produces the same effect in the arm it is laid in as yeast in a piece of dough; it ferments, and diffuses through the whole mass of blood the qualities with which it is impregnated. The pustules of the child in whom the artificial small-pox has been thus inoculated are employed to communicate the same distemper to others. There is an almost perpetual circulation of it in Circassia; and when unhappily the small-pox has quite left the country, the inhabitants of it are in as great trouble and perplexity as other nations when their harvest has fallen short...The Circassians are poor, and their daughters are beautiful, and indeed, it is in them they chiefly trade. They furnish with beauties the seraglios of the Turkish Sultan, of the Persian Sophy, and of all those who are wealthy enough to purchase and maintain such precious merchandise.[7]

Voltaire does not speculate on where the Circassians derived their technique from, though he reports that the Chinese have practiced it "these hundred years". The Turkish practice was presented to the Royal Society in 1714 and 1716, when the physicians Emanuel Timoni[8] and Giacomo Pylarini independently sent letters from Constantinople.[9]

Vial containing variolation material recovered from India during the early 1970s.

Variolation is documented in India from the eighteenth century, thanks to the 1767 account by J. Z. Holwell.[10] Holwell's extensive 1767 description included the following statement, on p.25, that points to the connection between disease and "multitudes of imperceptible animalculae floating in the atmosphere":

They lay it down as a principle, that the immediate cause of the smallpox exists in the mortal part of every human and animal form; that the mediate (or second) acting cause, which stirs up the first, and throws it into a state of fermentation, is multitudes of imperceptible animalculae floating in the atmosphere; that these are the cause of all epidemical diseases, but more particularly of the small pox.[10]

Holwell ascribes this account to his Brahman informants. However, such a theory has not yet been discovered in any Sanskrit or vernacular treatise.[11] Holwell's use of the word "animalculae" suggests that he may have been aware of the observations of Antonie van Leeuwenhoek made in about 1683. Other parts of Holwell's account, especially his use of the idea of "immediate" and "mediate" causation, and the concept of "fermentation," suggest a non-Indian source for his ideas outlined in this passage, which remain of great research interest.

Several historians have suggested that variolation may be older than the eighteenth century in India,[12] but historical evidence for this assertion is lacking. The widespread rumour since the nineteenth century that vaccination was documented in India before the discoveries of Edward Jenner can all be traced to propaganda tracts written in Sanskrit and the Indian vernaculars by colonial officers, in the pious hope of convincing Indians to accept the newly discovered Jennerian procedure.[13][14] A landmark anthropological study by Ralph Nicholas described the mid-twentieth century rituals of appeasement to Śītalā, the Indian goddess of smallpox, in Bengal.[15]

Holwell's account proves beyond doubt that smallpox inoculation was practised in India in the eighteenth century. Yet no Sanskrit or vernacular Indian sources yet discovered mention the practice. That such a major medical procedure would not be documented by the normally verbose members of the Brahman caste remains a historical puzzle. Some details of the practice described by Holwell (sniffing) suggest Chinese practice.

Early travellers to Ethiopia report that variolation was practiced by the Amhara and Tigray peoples. The first European to report this was Nathaniel Pearce, who noted in 1831, that it was performed by a debtera who would collect "a quantity of matter" from a person with the most sores from smallpox, then he "cuts a small cross with a razor in the arm" of his subject and put "a little of the matter" into the cut which was afterwards bound up with a bandage. Subsequent visitors who described this practice included W.C. Harris and Dr. Petit of the French scientific mission of 1839-1841.[16]

The knowledge of inoculating oneself against smallpox seems to have been known to West Africans, more specifically the Akan. A slave named Onesimus explained the inoculation procedure to Cotton Mather during the 18th century, he reported to have gotten the knowledge from Africa.[17]

The practice was introduced to England by Lady Mary Wortley Montagu. Lady Montagu's husband, Edward Wortley Montagu, served as the Britishambassador to the Ottoman Empire from 1716 to 1718. She witnessed firsthand the Turkish use of inoculation in Istanbul,[19] and was greatly impressed:[20] she had lost a brother to smallpox and bore facial scars from the disease herself. When a smallpox epidemic threatened England in 1721, she called on her physician to inoculate her daughter. She invited friends to see her daughter, including Sir Hans Sloane, the King's physician. Sufficient interest arose that Maitland gained permission to test inoculation at Newgate prison in exchange for their freedom on six prisoners due to be hanged, an experiment which was witnessed by a number of notable doctors.[21] All survived, and in 1722 the Prince of Wales' daughters received inoculations.[22]

The practice of inoculation slowly spread amongst the royal families of Europe, usually followed by more general adoption amongst the people.

The practice is documented in America as early as 1721, when Zabdiel Boylston, at the urging of Cotton Mather, successfully inoculated two slaves and his own son. Mather, a prominent Boston minister, had heard a description of the African practice of inoculation from his Sudanese slave, Onesimus, in 1706, and later from Timoni's report to the Royal Society,[23] but had been previously unable to convince local physicians to attempt the procedure.[24] Following this initial success, Boylston began performing inoculations throughout Boston, despite much controversy and at least one attempt upon his life. The effectiveness of the procedure was proven when, of the nearly three hundred people Boylston inoculated during the outbreak, only six died, whereas the mortality rate among those who contracted the disease naturally was one in six.[25] Boylston traveled to London in 1724. There he published his results and was elected to the Royal Society in 1726.

In France, considerable opposition arose to the introduction of inoculation, and it was banned by the Parlement. Voltaire, in his Lettres Philosophiques, wrote a criticism of his countrymen for being opposed to inoculation and having so little regard for the welfare of their children, concluding that "had inoculation been practised in France it would have saved the lives of thousands.".[27]

Inoculation grew in popularity in Europe through the 18th century. Given the high prevalence and often severe consequences of smallpox in Europe in the 18th century (according to Voltaire, there was a 60% incidence of first infection, a 20% mortality rate, and a 20% incidence of severe scarring),[28] many parents felt that the benefits of inoculation outweighed the risks and so inoculated their children.[29]

Two forms of the disease of Smallpox were recognised, now known to be due to two strains of the Variola virus. Those contracting Variola Minor had a greatly reduced risk of death – 1–2% – compared to those contracting Variola Major with 30% mortality. Infection via inhaled viral particles in droplets spread the infection more widely than the deliberate infection through a small skin wound. The smaller, localised infection is adequate to stimulate the immune system to produce specific immunity to the virus, while requiring more generations of the virus to reach levels of infection likely to kill the patient. The rising immunity terminates the infection. So the twofold effect is to ensure the less fatal form of the disease is the one caught, and to give the immune system the best start possible in combating it.

Inoculation in the East was historically performed by blowing smallpox crusts into the nostril. In Britain, Europe and the American Colonies the preferred method was rubbing material from a smallpox pustule from a selected mild case (Variola minor) into a scratch between the thumb and forefinger.[30] This would generally be performed when an individual was in normal good health, and thus at peak resistance. The recipient would develop smallpox; however, due to being introduced through the skin rather than the lungs, and possibly because of the inoculated individual's preexisting state of good health, the small inoculum, and the single point of initial infection, the resulting case of smallpox was generally milder than the naturally-occurring form, produced far less facial scarring, and had a far lower mortality rate. As with survivors of the natural disease, the inoculated individual was subsequently immune to re-infection.

Comparison of smallpox (left) and cowpox (right) inoculations 16 days after administration

In 1798, the English surgeon/scientist Edward Jenner published the results of his experiments and thus introduced the far superior and safer method of inoculation with cowpox virus, a mild infection that also induced immunity to smallpox. Jenner was not the first person to inoculate with cowpox nor the first to realize that infection with cowpox gave immunity to smallpox. However, he was the first to publish evidence that it was effective and to provide advice on its production. His efforts led to smallpox inoculation falling into disuse and eventually being banned in England in 1840.[31]

^In fact, the mortality rate of the Varoiola Minor form of smallpox then found in Europe was 1–3% as opposed to 30–50% for the Variola Major type found elsewhere; however, blindness, infertility, and severe scarring were common. Figures from "The Search for Immunisation", In Our Time, BBC Radio 4 (2006). [2]

^David V. Cohn, Ph.D. "Lady Mary Montagu". Founders of Science. Archived from the original on January 2, 2004. In England in 1718 she wrote to various influential persons urging inoculation and sent essays to subject to magazines. She had both her children inoculated - one in Turkey and one in England. Despite opposition from religious and medical groups, inoculation caught on. It was the primary defense against death and serious debilitation by smallpox for the next 80 years until the discovery of vaccination by Jenner.